$2-5k per month (after Apple's cut) from my Fitbit mobile app, Fitwatchr: https://itunes.apple.com/us/app/fitwatchr/id684005201?mt=8&i.... The unique value is Fitbit activity conversion to Weight Watchers points as well as a tougher but more rewarding calorie tracking that is based on real science (Mifflin equation). Effort on my part really depends on me/my current schedule as it's a side project. I admittedly have some bad reviews that boil down to getting users to understand how the calorie tracking works. Using Freshdesk (love it!) to answer 1-2 emails per day. It'll be 1 year working on this next month.